An optical semiconductor device and a method of manufacturing the same

Abstract: An optical semiconductor device is provided with a core layer (20) having a quantum well layer in which a film thickness gets thinner and the width gets wider from an inner region to an end portion in an optical waveguide region. Such a device can provide an improved efficiency of optical coupling between the active optical element and an optical fibre. The waveguide is made of a multi quantum well grown by selective area growth using a patterned mask geometry.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical semiconductor device and a method of manufacturing the same and more particularly to an optical semiconductor device having a high output semiconductor laser used for a light source of an information processing unit such as an optical printer and an optical disk unit, a semiconductor laser amplifier used for light amplification, an optical active element such as a semiconductor laser used in a transmitter of an optical communication apparatus or a photodiode used in a receiver thereof and an optical waveguide, and a method of manufacturing the same. 2. Description of the Prior Art With the advance of semiconductor communication techniques, production techniques of semiconductor lasers have also been improved, and research into integration of semiconductor laserswith other optical semiconductor elements are being made extensively in recent years. For example, an apparatus in which a DFB (distributed feedback) laser and a light modulator are integrated and an apparatus in which a DBR (distributed Bragg reflector) laser and a mode transducer (a beam size converter) are integrated, are available. A mode transducer is a mechanism for narrowing an output beam of a semiconductor laser originally having an output angle as wide as 30 to 40 degrees, and for facilitating optical coupling in the case in which a semiconductor laser and an optical fiber are formed into a module. In a semiconductor laser, the more intense the optical confinement of an optical waveguide is, i.e., the smaller a light spot diameter is, the smaller an oscillation threshold becomes, and a luminous efficiency is improved. As the light spot diameter gets smaller, however, coupling with the optical fiber becomes more difficult. Further, a semiconductor laser requiring a high output, such as a laser for exciting a fiber type optical amplifier using an optical fiber doped with erbium or a semiconductor laser for writing information in a optical disk, suffers from the problem that an optical power density rises at a laser end face and damage to the end face is liable to be caused. Furthermore, a semiconductor laser amplifier may have the drawback that the optical output is saturated easily when beam confinement is intense. Photodiodes that have a rapid speed of response and a high quantum efficiency are required, and furthermore, photodiodes that can be formed into a thin shape and that can permit convenient electrical wiring are demanded. An end face incidence waveguide type photodiode is available as a photodiode which meets these requirements. In this waveguide type photodiode, the more intense the beam confinement is, the shorter the waveguide is made. In this way, an internal pn junction capacity thereof is reduced, thus making high-speed response possible. Moreover, reactive components of light absorption such as free carrier absorption of a cladding layer are reduced so that the quantum efficiency is increased, thus improving sensitivity. Under such circumstances, a semiconductor laser, a semiconductor laser amplifier, and a photodiode, having an optical waveguide in which beam confinement is intense inside and beam confinement is weak at an end face,are demanded in the fields of optical communication and optical information processing. To this end, waveguides for converting an optical beam diameter composed of a semiconductor have been proposed as shown in FIG. 1A to FIG. 1C. The semiconductor waveguide shown in FIG. 1A is disclosed in [1] the Institute of Electronic Information Communications in Japan, National Autumn Meeting 1992, Lecture Number C-201 for example. The semiconductor waveguide shown in FIGs. 1B and 1C has been proposed in [2] the Institute of Electronic Information Communications in Japan, National Autumn Meeting 1992, Lecture Number C-202 for instance. In [3] T.L. KOCH et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 2, NO. 2, 1990, another semiconductor waveguide is proposed in a mode transducer integrated Fabry-Pérot semiconductor laser (hereinafter referred to also as an FP-LD) having a waveguide for converting an optical beam diameter. The optical beam diameter converting waveguide shown in FIG. 1A has an InGaAsP core layer 3 surrounded by an InP substrate 1 and an InP cladding layer 2 at top and bottom and left and right. The lateral dimension (width) of the InGaAsP core layer 3 is wide at one end face and gets narrower with greater proximity to the opposite end face, and the optical beam diameter spreads in the lateral direction with greater proximity to the said opposite end face (where the width of the core layer is narrowed) in accordance with the variation of the core-layer width. The thickness of the InGaAsP core layer 3 is uniform, and the plane pattern thereof is formed by lithography technique using an exposure mask. Thus, the semiconductor waveguide is effective for converting an optical beam diameter in a lateral direction, but the optical beam diameter in a longitudinal direction (a thickness direction of the layer) is not converted. Now, in an optical waveguide using a semiconductor, since the optical beam diameter in the thickness direction of the core layer is generally smaller than that in the width direction thereof, conversion of the optical beam diameter in the longitudinal direction is important for the improvement of the optical coupling efficiency with an optical fiber, an optical semiconductor element or the like. Since the optical beam diameter in the thickness direction of the layer is not converted in the semiconductor waveguide for converting the optical beam diameter shown in FIG. 1A, a significant effect cannot be expected as regards the improvement of the optical coupling efficiency between the semiconductor optical waveguide and an optical fiber or the like. In order to increase the coupling efficiency it is conceivable to form a semiconductor waveguide, used to convert the optical beam diameter, and a waveguide-shaped photodiode into an integral construction. Since the thickness and the composition of the core layers become uniform in the optical axis direction, however, the end face of the semiconductor waveguide also becomes a light absorption region and the optical loss is increased. Furthermore, since a PN junction is exposed at the end face, a dark current is increased. When the dark current is more or less increased, there is a problem that a signal-to-noise ratio may deteriorate undesirably, especially in a case in which very high sensitivity is required. As against the above, the semiconductor waveguide for converting the optical beam diameter shown in FIGS. 1B and 1C has a construction that the optical beam diameter is converted in the thickness direction of the film. This semiconductor waveguide has a first InP cladding layer 5 laminated on an InP substrate 4, a multi-quantum well (MQW) layer 6 composed of an InP well and an InAℓAs barrier formed thereon and a second InP cladding layer 7 formed on the MQW layer 6. Further, an InGaAsP core layer 8 is formed in the MQW layer 6, and the film thickness at one end thereof is reduced in the MQW layer 6. Besides, the MQW layer 6 serves as a cladding layer for the core layer 8 and as a core layer for the first and the second InP cladding layers 5 and 7. The core layer 8 is formed both in a gain region 9 and a mode conversion region 10. Further, the core layer 8 in the mode conversion region 10 is formed in a tapered shape in the thickness direction, and gets thinnerwith increasing distance from the gain region 9. The light advancing in such a semiconductor waveguide is confined in the MQW layer 6 and further confined more intensely in the cladding layer 7. The optical beam diameter is converted at a portion where the film thickness of the core layer 8 is changed. Further, since the light excited in the MQW layer 6 is confined more weakly in the mode conversion region 10 than in the gain region 9, a near field pattern at a taper bottom end portion is spread. As a result, a far field pattern which is a diffracted pattern of the near field pattern is contracted. Accordingly, an output angle of a beam emitted from the taper bottom end is narrowed, which makes coupling with an optical fiber easier. Three cone-shaped patterns in FIG. 1C show intensity distribution of a photoelectric field. Now, the core layer 8 shown in FIG. 1C has a structure in which an InGaAsP layer 8a and an InP layer 8b are laminated alternately as shown in an enlarged view led out from the main view with a dashed line, and the InGaAsP layers 8a and 8b are subject to stepwise patterning using the InP layer 8b as an etching stop layer, thus varying the film thickness of the core layer 8 in stepwise manner. Since this construction requires patterning to be repeated many times to vary the film thickness of the core layer 8, the production rate (throughput) is lowered. When such a semiconductor waveguide is integrated in a monolithic manner with a light emitting element, active layers of the core layer 8 and the light emitting element become the same. Therefore, a defect is liable to be produced in the crystal of the active layer of the light emitting element in processing for varying the film thickness of the core layer 8, thuspresenting a risk that the characteristics of the light emitting element will be adversely affected. Further, when such a semiconductor waveguide and a light receiving element are integrated in a monolithic manner, a defect is also liable to be produced in the absorption layer of the light receiving element, thus also causing a risk that the characteristics of the light receiving element will be adversely affected. The FP-LD described in the above-mentioned citation [3] is structured by forming a first InP cladding layer 102, a waveguide layer 103, a multiple quantum well (MQW) active layer 104 and a second InP cladding layer 105 one upon another on an InP substrate 101 as shown in FIG. 2. The MQW active layer 104 is structured of an InGaAs well layer and an InGaAsP barrier layer, and is formed only in a gain region 110. Further, the waveguide layer 103 is formed in both the gain region 110 and a mode conversion region 111. Besides, a reference numeral 106 represents a contact layer formed on the second InP cladding layer 105, and 107 represents an etching stop layer formed in the second cladding layer. The waveguide layer 103 in the mode conversion region 111 is formed in a tapered form in the thickness direction, and gets thinner with greater distance from the gain region 110. The waveguide layer 103 has a structure in which an InGaAsP layer 103a and an InP layer 103b are laminated alternately. Further, the InP layer 103b is used as an etching stop layer, and the InGaAsP layer 103a and the InP layer 103b are subjected to patterning to form a step form while changing the etching proof mask a plurality of times, thus forming the film thickness of the waveguide layer 103 in the mode conversion region in a tapered form. As to the light excited in the MQW active layer 104 and conducting through the waveguide layer 103, the near field pattern thereof at the taper bottom end portion spreads since optical confinement is weaker in the mode conversion region 111 than that in the gain region 110, thus resulting in that the far field pattern that is a diffracted pattern of the near field pattern is narrowed. Accordingly, the output angle of a beam emitted from the taper bottom end is narrowed, thus making coupling with an optical fiber easier. Now, the waveguide layer 103 forming a resonator has a construction in which the number of layers becomes less with greater proximity to the output end, and the tapered form of the waveguide layer 103 is obtainable by changing the etching-proof mask a plurality of times. Therefore, this construction is liable to include a crystal defect and adverse effects on the characteristics of a light emitting element are liable to be produced. Further, it is important, when a tapered waveguide layer is integrated in a semiconductor laser, to ensure that the waveguide located in the mode conversion region does not act as an absorbing medium for an oscillation light. Since the active layer 104 in the gain region 110 and the tapered waveguide layer 103 in the mode conversion region 111 are different layers and formed of almost the same composition in the construction described in the above-mentioned citation [3], a part of the laser light is liable to be absorbed in the portion near the gain region 110 of the tapered waveguide layer 103. Accordingly, lowering of an optical output and a slope efficiency (a differential value of a leading edge of a current to light intensity characteristic curve) is unavoidable as compared with a laser element in which the mode converter is not integrated. In particular, since the tapered waveguide is located in a resonator in the FP-LD described in the citation [3], even the fundamental characteristics of the semiconductor laser can be affected, and a threshold current in pulse measurement rises up to 70 mA in an element having double end cleavage. Moreover, no report has been made that continuous oscillation has occurred at room temperature in an element having double end cleavage. Further, other examples in which a semiconductor laser and a tapered waveguide are integrated has been disclosed in [4] Japanese Patent Provisional Publication No. SH063-233584 and [5] Japanese Patent Provisional Publication No. SH064-53487. In the constructions thereof, the laser active layer and the tapered waveguide are also formed of the same composition, and the absorption loss in the taper region is great. In order to avoid the absorption loss, it is required to inject a large current into the whole tapered waveguide, too. In the light intensity-current characteristic in the provisional publication [4], only a threshold current of 42 mA and a differential quantum efficiency of 0.15 mW/mA were obtained, as is apparent from the description. Further, since the tapered form of the waveguide layer is obtained by means of an etching method, it becomes difficult to form the tapered form sufficiently uniformly from one device to the next, thus presenting the risk that the yield may be lowered in addition to poor controllability of the beam spot configuration. Moreover, a crystal defect is liable to be produced in the waveguide layer forming the resonator and adverse effects on the characteristics of a light emitting element are liable to be produced. Furthermore, the waveguide layer is composed of a single material, and does not have an MQW layer in a tapered form such as described in the citation [3]. Besides, an optical semiconductor device having a tapered form in a lateral direction is described in the provisional publication [3], and processing in the submicron order is required at the bottom end portion of the tapered waveguide. Hence, it is difficult to produce the same tapered form with good reproducibility because of the structure thereof. Another optical semiconductor device in which an active optical element and a waveguide are formed integrally is described in a paper by Cockerill et al., "Monolithic Integration of Strained-Layer InGaAs - GaAs - AlGaAs Quantum-Well Laser with a Passive Waveguide by Selective-Area MOCVD", IEEE Photonics Technology letters, vol. 5, No. 4, April 1993, pp. 448-450. This device may be considered to include: a support structure having a semiconductor substrate; and a quantum well structure formed on the said support structure and having a stack of layers including at least one well layer and two barrier layers sandwiching the well layer, the said quantum well structure including an active optical region, forming at least part of the said active optical element, and also including a waveguide region having a first end adjacent to the said active optical region and a second end adjacent to an interface portion of the device at which an output optical beam is emitted from the device, or an input optical beam is received by the device, when the device is in use, the said waveguide region of the quantum well structure forming at least part of a core layer of the said waveguide and serving to couple the active optical element optically to the said interface portion when the device is in use, there being upper and lower cladding layers above and below the core layer respectively. A method of manufacturing the device, also disclosed in the same paper, may be considered to include: forming a masking film of dielectric material on an upper main face of a support structure including a semiconductor layer; patterning the masking film so as to expose respective first and second regions of said semiconductor layer of the said support structure, the said first region being elongate and the said second region having a first end adjoining one end of the said first region and a second end, of greater width than the said first region, adjacent to an intended interface portion of the finished device at which, in use of the said finished device, an output optical beam is to be emitted from the device or an input optical beam is to be received by the device; and forming layers successively over the first and second regions so as to provide over both regions a quantum well structure having a stack of the layers including at least one well layer and two barrier layers sandwiching the well layer, and so as to provide over at least the said second region upper and lower cladding layers above and below the said quantum well structure respectively, at least the said well layer being formed by a selective area growth method so that its thickness is greater over the said first region than at the said second end of the said second region. In this device, the quantum well structure is relatively thick in the active optical region in which a laser is formed and relatively thin in the waveguide region. There is a step change in thickness in a short boundary region between the active optical region and the waveguide region. EP-A-0495202 discloses a passive beam-converter arrangement for coupling a semiconductor optical element such as a laser to an optical conductor such as a monomode fiber. The arrangement has a first optical waveguide embedded in a second, larger optical waveguide. The first optical waveguide tapers as it extends into the second waveguide and ends at a distance from the end face of the second waveguide so that an optical wave propagating through the first waveguide spreads as a consequence of the reducing thickness of the first waveguide, until guidance of the wave is taken over by the second waveguide. According to a first aspect of the present invention there is provided an optical semiconductor device in which an active optical element and a waveguide are formed integrally, the device including: a support structure having a semiconductor substrate; and a quantum well structure formed on the said support structure and having a stack of layers including at least one well layer and two barrier layers sandwiching the well layer, the said quantum well structure being formed in an active optical region of the said active optical element, and also being formed in a waveguide region that has a first end adjacent to the said active optical region and that has a second end adjacent to an interface portion of the device at which an output optical beam is emitted from the device, or an input optical beam is received by the device, when the device is in use, the quantum well structure in the said waveguide region forming at least part of a core layer of the said waveguide and serving to couple the active optical element optically to the said interface portion when the device is in use, there being upper and lower cladding layers above and below the core layer respectively; characterised in that the thickness of the said well layer decreases gradually over the length of the said waveguide region from the said first end thereof to the said second end thereof. In such an optical semiconductor device the active optical element (such as a semiconductor laser, a light emitting diode or a photodiode) and the optical waveguide are formed in a monolithic manner, and the diameter of an optical beam in a thickness direction of the core layer at an end face of the optical waveguide can be suitably large for good coupling efficiency to an external optical conductor. Furthermore, the waveguide, which serves as a mode converter, has reduced optical absorption, and the active optical element can, for example, perform stable laser oscillation at a low threshold. A part of the active optical element is constituted by one part (the active optical region) of the quantum well structure which has for example a stripe form, whilst another part (the waveguide region)of the same quantum well structure constitutes a part of the waveguide, and the quantum well structure in the waveguide region is made thinner with greater distance from the active optical region without changing the number of layers, thus converting the diameter of the optical beam. In one embodiment in which light is outputted from the active optical element, optical confinement gets weaker in the thickness direction of the core layer as the light goes toward the end portion (interface portion). Thus, the optical beam diameter gets larger. Further, in an embodiment in which light is received at the interface portion (input end of the optical waveguide) optical confinement becomes more intense as the light travels toward the active optical element, and the optical beam diameter gets smaller as it travels to reach the active optical element. As a result, the efficiency of optical coupling between the optical active element and the optical fiber is improved by going through the optical waveguide. Moreover, since the optical waveguide and the active optical element are formed as one body, positioning from the outside becomes unnecessary and so the optical coupling loss is reduced. Further, an energy band gap of the core layer of the optical waveguide becomes larger with greater distance from the active optical element. Therefore, when the active optical element is a semiconductor laser or a semiconductor laser amplifier, the light having a laser-oscillated wavelength is not absorbed significantly by the optical waveguide and so the absorption loss in the optical semiconductor device is reduced. Furthermore, since the difference in refractive index between the core layer and the cladding layer of the waveguide becomes smaller with greater distance from the active optical element, the optical beam diameter becomes larger at a distant position from the active optical element and becomes smaller at a near position therefrom. As a result, the coupling efficiency between the optical active element and the optical fiber is further improved by having the optical waveguide lie therebetween. The cladding layer of the optical waveguide is preferably formed with a non-doped semiconductor, so that light absorption by free carriers is reduced, thus controlling attenuation of light intensity. Further, when a cladding layer of the optical waveguide and a cladding layer of the optical active element are separated from each other by a groove the parasitic capacity of the active optical element can be reduced. The width of the waveguide core layer preferably increases gradually in the direction away from the active optical element so that a mode of the light expands in the width direction through the core layer of the optical waveguide, and hence the optical beam diameter in the width (lateral) direction can also be increased at the interface portion. However, even when the width of the core layer is uniform, optical confinement not only in the direction perpendicular to the core layer but also in a direction parallel to the core layer also becomes weaker as the core layer gets thinner. Therefore, although the degree of the effect is comparatively small, a similar beam-width-increasing effect is obtainable. The thickness of the quantum well structure in the waveguide region may be twice or more at the input end (first end) of the region than at the output end (second end). According to the foregoing, the shape of the beam spot emitted from the quantum well structure becomes a circle or a shape very close to a circle, and moreover, the beam output angle thereof falls within the range from 20 degrees to 10 degrees. Hence, the coupling efficiency with an optical fiber or the like is improved by a large margin. Furthermore, in this construction, the waveguide region of the quantum well structure, forming part of the waveguide for mode conversion, is thinner than the active optical region of the quantum well structure forming part of the active optical element and a ground level of the quantum well is higher in the waveguide for mode conversion than that in the active optical element. Thus, a light absorption wavelength edge in the waveguide for mode conversion at the output end portion (interface portion) has a shorter wavelength than the oscillation wavelength in the active optical element. With this, the light conducted in the quantum well structure to the output end portion is not absorbed in the waveguide (mode conversion) region. Further, a circular beam spot can be formed by forming the quantum well structure in a stripe shape and setting the ratio of minimum values of both end widths of the stripe-shaped quantum well structure to 0.8 to 1.2, thus making optical coupling with an optical fiber or the like easier. Now, since the film thickness of the quantum well structure gets thinner gradually from the end portion of the active optical region (gain region) to the light output end of the waveguide for mode conversion, there is a risk that a part of the waveguide for mode conversion in the vicinity of the gain region may become a saturable absorption layer, thus inflicting a serious loss. In a preferred embodiment of the invention, however, a current is applied locally by providing an electrode or extending a contact layer in a boundary region near the gain region in the waveguide for mode conversion, so that the boundary region becomes transparent reliably and significant light absorption does not occur. With this, it is possible to reduce the light absorption in the waveguide for mode conversion, thus making it possible to perform stable laser oscillation at a low threshold. The electrode formed in the boundary region near the gain region in the waveguide for mode conversion may be obtained either by extending the electrode in the gain region or by forming it separately from the electrode in the gain region, but it is easier to control the light absorption quantity by using a separate electrode. According to a second aspect of the present invention there is provided a method of manufacturing an optical semiconductor device in which an active optical element and a waveguide are formed integrally, including: forming a masking film of dielectric material on an upper main face of a support structure including a semiconductor substrate; patterning the masking film so as to expose respective first and second regions of semiconductor material of the said support structure, the said first region corresponding to an active optical region of the said active optical element and being elongate, and the said second region corresponding to a waveguide region and increasing gradually in width from a first end thereof adjoining one end of the said first region to a second end thereof adjacent to an intended interface portion of the finished device at which, in use of the said finished device, an output optical beam is to be emitted from the device or an input optical beam is to be received by the device; and forming layers successively over the first and second regions so as to provide over both regions a quantum well structure having a stack of the layers including at least one well layer and two barrier layers sandwiching the well layer, the quantum well structure over the said second region forming at least part of a core layer of the said waveguide, and so as to provide over at least the said second region upper and lower cladding layers above and below the said quantum well structure respectively, at least the said well layer being formed by a selective area growth method so that its thickness decreases gradually over the length of the said second region from the said first end thereof to the said second end thereof. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A, discussed hereinbefore, is a perspective view showing a first example of a conventional optical beam diameter conversion waveguide; FIGs. 1B and FIG. 1C, discussed hereinbefore, are a perspective view and a sectional view respectively, showing a second example of a conventional optical beam diameter conversion waveguide; FIG. 2, discussed hereinbefore, is a sectional view showing a conventional mode converter integrated Fabry-Perot type semiconductor laser; FIGS. 3A to 3K are perspective views showing a manufacturing process of a first optical semiconductor device according to an embodiment of the present invention; FIG. 4A and FIG. 4B are sectional views showing a part of the manufacturing process of the first optical semiconductor device according to an embodiment of the present invention; FIG. 5 is a perspective sectional view showing the first optical semiconductor device according to an embodiment of the present invention; FIG. 6 is a diagram showing, in the manufacturing process of the first optical semiconductor device according to an embodiment of the present invention, the relationship between the size of window width of a mask pattern and the growth speed of the semiconductor in the window; FIG. 7A and FIG. 7B are diagrams each showing an energy band gap of an MQW layer of the first optical semiconductor device according to an embodiment of the present invention; FIG. 8 is a diagram showing the relationship between the thickness of the core layer of an optical beam diameter conversion waveguide and a beam diameter in the first optical semiconductor device according to an embodiment of the present invention; FIG. 9 is a diagram showing the relationship between optical electric field strength of a core layer and optical electric field strength of an active layer in the first optical semiconductor device according to an embodiment of the present invention; FIG. 10A is a characteristic diagram showing the relationship between the thickness of a well and a barrier of an MQW layer and variation of an energy band gap in the first optical semiconductor device according to an embodiment of the present invention, and FIG. 10B is a characteristic diagram showing the relationship between a wavelength and a coefficient of light absorption when the thickness of the MQW layer is changed; FIG. 11 is a coupling loss characteristic diagram when the first optical semiconductor device according to an embodiment of the present invention is connected to an optical fiber; FIG. 12A is an energy band diagram showing an example of a conventional MQW, and FIG. 12B is a coupling loss characteristic diagram when the semiconductor laser having the conventional MQW is connected to an optical fiber; FIG. 13 is a perspective view showing an example of a connecting device of the first optical semiconductor device according to an embodiment of the present invention and an optical fiber; FIG. 14 is a perspective view showing a mask used in forming a second optical semiconductor device of an embodiment of the present invention; FIG. 15 is a sectional view showing the second optical semiconductor device of an embodiment of the present invention; FIG. 16 is a perspective view showing a mask used in mass production of optical semiconductor devices according to an embodiment of the present invention; FIG. 17A and FIG. 17B are diagrams each showing an energy band gap of an MQW layer of a third optical semiconductor device according to an embodiment of the present invention; FIG. 18A and FIG. 18B are perspective views each showing a part of a manufacturing process of the third optical semiconductor device according to an embodiment of the present invention; FIG. 19 is a coupling loss characteristic diagram when the third optical semiconductor device according to an embodiment of the present invention is connected to an optical fiber; FIG. 20 is a sectional view showing a fourth optical semiconductor device according to an embodiment of the present invention; FIG. 21 is a sectional view showing a fifth optical semiconductor device according to an embodiment of the present invention; FIG. 22 is a sectional view showing another construction of the fifth optical semiconductor device according to an embodiment of the present invention; FIG. 23 is a sectional view showing a sixth optical semiconductor device according to an embodiment of the present invention; FIG. 24 is a perspective sectional view of a seventh optical semiconductor device according to an embodiment of the present invention; FIG. 25A and FIG. 25B are perspective views each showing the manufacturing process of the seventh optical semiconductor device according to an embodiment of the present invention; FIG. 26A to FIG. 26C are side sectional views each showing a part of the manufacturing process of the seventh optical semiconductor device according to an embodiment of the present invention; FIG. 27A to FIG. 27C are front sectional views each showing a part of the manufacturing process of the seventh optical semiconductor device according to an embodiment of the present invention; FIG. 28A and FIG. 28B are diagrams each showing an energy band of a quantum well construction in the seventh optical semiconductor device according to an embodiment of the present invention; FIG. 29 is a sectional view showing a preferable configuration of a p-side electrode in the seventh optical semiconductor device according to an embodiment of the present invention; FIG. 30 is a characteristic diagram showing the relationship between the current and the optical output in the seventh optical semiconductor device according to an embodiment of the present invention; FIG. 31A and FIG. 31B are light intensity distribution diagrams in a horizontal direction and a vertical direction of a far field pattern of the seventh optical semiconductor device according to an embodiment of the present invention; FIG. 32 is a characteristic diagram showing the relationship between the taper length and a spot diameter of a near field pattern of the seventh optical semiconductor device according to an embodiment of the present invention; FIG. 33 is a characteristic diagram showing the relationship between a film thickness ratio and a beam output angle of an optical semiconductor device according to a seventh embodiment of the present invention; FIG. 34 is a sectional view showing a model of the waveguide used when the characteristic shown in FIG. 33 is computed; and FIG. 35 and FIG. 36 are sectional views showing variation in arrangement of an electrode of the seventh optical semiconductor device according to an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS (The first embodiment) FIG. 3A to FIG. 3K are perspective views showing a manufacturing process of a first optical semiconductor device according to an embodiment of the present invention. FIG. 4A and FIG. 4B are partial sectional views of the manufacturing process, and FIG. 5 is a partly cut out perspective view of a completed optical semiconductor device. Next, the process to reach a state shown in FIG. 3A will be described. First, a diffraction grating 13 is formed by an interference exposure method in a region including a semiconductor laser forming region 12 on a surface of an n-InP substrate 11. The diffraction grating 13 is formed of irregularities repeated at a constant pitch in the waveguide direction. Then, n-InGaAsP whose composition is specified by a wavelength λg of 1.5 µm showing the composition is accumulated in a thickness of 150 nm by an epitaxial growth method, thereby to form a guide layer 14. Furthermore, an n-InP buffer layer (cladding layer) 15 is formed thereon in a thickness of 10 nm to produce a support structure. Thereafter, a dielectric masking film 16 such as SiO 2 or Si x N y is formed on the support structure by a thermal CVD method and an opening is formed by a lithography method in the dielectric film 16 at the semiconductor laser forming region 12 and a waveguide forming region 17, thereby to form a spatula-shaped window 18. The configuration of the semiconductor laser forming region 12 is rectangle having a longer side in the waveguide direction. On the other hand, the waveguide forming region 17 has a shape obtained by combining a trapezium portion, in which the width thereof widens continuously at a constant angle with greater distance from the end portion of the semiconductor laser forming region 12, and a rectangular portion that is in contact with the base of the trapezium portion. The width W 1 of the window 18 at the end portion of the semiconductor laser forming region 12 (first end of the waveguide region) is 15 to 20 µm, and the width W 2 of the base of the trapezium portion (second end) of the waveguide forming region 17 is 150 to 200 µm. Next, as shown in FIG. 3B, a first InGaAsP guide layer 19 is grown epitaxially, and an MQW layer (quantum well structure) 20 composed of three InGaAs well layers and InGaAsP barrier layers having a wavelength λg of 1.3 µm sandwiching the well layers therebetween, a second InGaAsP guide layer 21, a p-InP cladding layer 22 and an n-InGaAsP etching stop layer 23 are grown one after another epitaxially. Besides, the layers 19 to 23 may be formed after the surface of the n-InP cladding layer 15 exposed from the window 18 is etched slightly. This etching is for purifying the surface of the cladding layer 15. These semiconductor layers 19 to 23 are not grown on the dielectric film 16 which has no growth nucleus, but are grown in the region of the window 18 selectively. Moreover, since the crystal growth speed becomes lower as the pattern width W of the window 18 gets larger, the film thickness of these semiconductor layers 19 to 23 depends on the pattern width W of the window 18, and the final film thickness becomes thin at the wide end (W 2 ) as shown in FIG. 6. Accordingly, the respective semiconductor layers 19 to 23 laminated on the buffer layer 15 through the window 18 are thickest in the semiconductor laser forming region 12 and get thinner gradually toward the bottom end portion of the waveguide forming region 17 from the end portions thereof. For example, when it is assumed with respect to the MQW layer 20 that the thickness of an InGaAs well layer 20a and the thickness of an InGaAsP barrier layer 20b in the semiconductor laser forming region 12 are 6 nm and 16 nm, respectively, the thickness of the InGaAs well layer 20a and the thickness of the InGaAsP barrier layer 20b at the bottom end portion of the waveguide forming region 17 become 1.5 nm and 4 nm, respectively. Therefore, the energy band thereof appears as shown in FIG. 7A in the semiconductor laser forming region 12 and appears as shown in FIG. 7B in the waveguide forming region 17, respectively. In the above example, the film thickness of the guide layers 19 and 21 at the bottom end portion of the waveguide forming region 17 is set to 10 nm, and the film thickness of the p-InP cladding layer 22 and the film thickness in the semiconductor laser forming region 12 of the InGaAsP etching stop layer 23 areset to 500 nm and 10 nm, respectively. Next, another dielectric film is formed by a thermal CVD method after the dielectric film 16 is removed and issubjected to patterning by a lithography technique, thereby to form a mask pattern 24 covering a part of the top of the etching stop layer 23 as shown in FIG. 3C. This mask pattern 24 has a configuration like a spatula exposing edge portions on both sides of the top of the etching stop layer 23. For example, it is rectangular in shape (having the width W 3 of 1.5 pm) in the semiconductor laser forming region 12, and has a configuration obtained by putting together a trapezium, having a width W 4 of 8 um at the base, and a quadrilateral, extending from the trapezium base without varying in width, in the waveguide forming region 17. Next, as shown in FIG. 3D, etching is applied continuously up to the etching stop layers 23 located on both sides of the mask pattern 24 and the top of the n-InP substrate 11, thus defining the final width of the semiconductor laser and the waveguide for converting optical beam diameter. Namely, the width of the active layer of the semiconductor laser becomes 1.5 µm, and that of the bottom end of the waveguide becomes 8 µm. Thereafter, an InP high resistance layer 25 doped with iron is formed by an epitaxial method on the n-InP substrate 11 exposed on both sides of the mask pattern 24 as shown in FIG. 3E. Then, as shown in FIG. 3F, an n-InP current block layer 26 and an InGaAsP etching stop layer 27 are grown epitaxially on the high resistance layer 25. Next, after the mask pattern is removed as shown in FIG. 3G, a non-doped InP cladding layer 28 is grown epitaxially so that the film thickness at the bottom end portion of the waveguide forming region 17 reaches 2.5 µm or more as shown in FIG. 3H. FIG. 4A is a sectional view taken along a line I-I in FIG. 3H. Here, the reason why non-doped InP is used instead of p-InP is to reduce light absorption by free carriers in the optical beam diameter conversion waveguide. Furthermore, as shown in FIG. 3I, a dielectric film 29 is formed, patterning is applied to this film 29 thereby to form a window 30 in the semiconductor laser forming region 12 and thereabout, and the InP cladding layer 28 exposed from the window 30 is etched selectively. In this case, the etching depth is controlled by the InGaAsP etching stop layers 23 and 27. Then, after the etching stop layers 23 and 27 exposed from the window 30 are removed selectively by using an etchant which does not etch the current block layer 26, a p-InP cladding layer 31 is grown epitaxially on the p-InP cladding layer 22 through the window 30 as shown in FIG. 3J, thereby to aim at levelling the cladding layer 31 and the non-doped InP cladding layer 28. Thereafter, as shown in FIG. 3K, patterning is applied to the non-doped InP cladding layer 28 and the p-InP cladding layer 22 below the layer 28, the InGaAsP etching stop layers 23 and 27 and the current block layer 26, thereby to form a groove 32 for separating the semiconductor laser forming region 12 and the waveguide forming region 17 from each other. In the patterning, the guide layer 21 is finally made to function as an etching stop layer. Furthermore, an n-side electrode 33 is formed on the under surface of the n-InP substrate 11, and a p-side electrode 34 is formed on the p-InP cladding layer 31 in the semiconductor laser forming region 12 at the same time. With this, the forming process of a monolithic BH type semiconductor laser 35 and an optical beam diameter conversion waveguide 36 such as those shown in FIG. 4B and FIG. 5 is completed. Besides, FIG. 4B is a sectional view taken along a line II-II in FIG. 3K. The width of the MQW layer 20 which becomes a core layer may be made constant. In this case, it is sufficient to set the width of the mask 24 in FIG. 3D constant, e.g., to W 3 . The above-mentioned semiconductor laser 35 oscillates by applying a current across the n-side electrode 33 and the p-side electrode 34, and, when the laser light is outputted to the optical beam diameter conversion waveguide 36, the laser light advances while being confined within the MQW layer 20. At this time, in the MQW layer 20 in the optical beam diameter conversion waveguide 36, the film thickness gets thinner with increasing distance from the semiconductor laser 35, and the optical beam diameter in the film thickness direction increases. This is due to the fact that in the MQW layer 20, which is the core layer of the waveguide 36, optical confinement gets weaker as the film thickness D thereof gets thinner. Thus, as shown in FIG. 8, the light emitted from the MQW layer 20 (having a thickness of D 1 ) of the semiconductor laser 35 spreads out in the film thickness direction as it propagates towards the bottom end portion (having a thickness of D 2 ) of the optical beam diameter conversion waveguide 36. Namely, as shown in FIG. 9, whereas a photoelectric field intensity distribution P 1 in the semiconductor laser 35 has a sharp peak of intensity in the MQW layer 20, a photoelectric field intensity distribution P 2 at the bottom end portion of the optical beam diameter conversion waveguide 36 has a broad spread, which is almost the same as that of a light intensity distribution P 3 of an optical fiber 37 connected to the end portion thereof. Thus, it is ensured that the coupling efficiency between the semiconductor laser 35 and the optical fiber 37 is improved by the optical beam diameter conversion waveguide 36 described above. Further, in the MQW layer 20, the energy band gap between a quantum level on the valence band side and a quantum level on a conductive band side becomes larger as the well layer gets thinner, and moreover, the energy band gap between the valence band and the conductive band becomes larger (Eg 1 <Eg 2 ) as the film thickness gets thinner as shown in FIG. 7A and FIG. 7B. This is apparent from the relationship between the thickness D of the semiconductor layer and the variations aEg of the energy band gap such as shown in FIG. 10A. When the energy gap becomes larger, an edge of the absorption vs. wavelength characteristic for the end face of the optical beam diameter conversion waveguide 36 (having the thickness D 2 ) falls on the side shorter than a wavelength λ 0 of the light outputted from the semiconductor laser 35 as shown in FIG. 10B. Therefore, the optical beam diameter conversion waveguide 36 does not absorb the light. As a result, attenuation of the light intensity in the optical beam diameter conversion waveguide 36 is reduced, and the intensity of the light outputted to the optical fiber 37 is increased. Moreover, when an InGaAs well layer 20a and an InGaAsP barrier layer 20b are compared with each other, it is found that the variation of the film thickness of the well layer is larger. The refractive index of the MQW can be regarded as the average of the refractive indices, taking into account the thicknesses of the well layer and the barrier layer. Accordingly, the MQW layer 20 at the bottom end of the optical beam diameter conversion waveguide 36 has a refractive index smaller than that of the MQW layer 20 of the semiconductor laser 35, and the difference in the refractive index from the cladding layers 22 and 15 above and below the MQW layer 20 is reduced, thus enlarging the optical beam diameter. On the other hand, in the optical beam diameter conversion waveguide 36, the width of the MQW layer 20 becomes wider towards the output end. Therefore, the beam diameter of the light confined therein also gets wider in the width (lateral) direction in accordance with the change of the width thereof. Besides, when the MQW layer 20 gets thinner, optical confinement not only in the film thickness direction but also in the direction perpendicular to the film thickness (the lateral direction) gets weaker even if the width of the MQW layer 20 is fixed. Accordingly, the optical beam diameter spreads, thus improving the coupling efficiency. Next, the coupling loss will be calculated in the case in which an optical fiber 37, having a core diameter of 10 µm, and the bottom end of the above-described optical beam diameter conversion waveguide 26 are directly placed opposite each other. The results are shown in FIG. 11. In FIG. 11, the horizontal axis (abscissa) represents an offset between the respective central axes of the optical fiber 37 and the waveguide 36, and the vertical axis (ordinate) represents the coupling loss between the fiber and waveguide. Further, in FIG. 11, a solid line shows the coupling loss in a vertical direction (the film thickness direction of the MQW layer) and a dashed line shows the coupling loss in a horizontal direction (the width direction of the MQW layer). As against the above, when the coupling loss was investigated in a case in which a conventional semiconductor laser, having such a MQW construction as shown in FIG. 12A, and an optical fiber were placed opposite each other directly, a result such as that shown in FIG. 12B was obtained. With this, it has been found that the coupling loss is less by 3dB in the present embodiment. Now, since it is desired to couple the above-mentioned optical semiconductor device with the optical fiber 37 without requiring any adjustment, an example thereof is shown in FIG. 13. In FIG. 13, a silicon wafer 41 is partitioned into a plurality of sections, and bonding pads 42 for fixing the optical semiconductor device shown in FIG. 5 and a V-shaped groove 43 for mounting the bottom end portion of the optical fiber 37 are formed in each section. Further, in a region where the optical semiconductor device is to be mounted, an extending electrode 44 to be connected to the p-side electrode 34 of the semiconductor laser 35 is formed. The V-shaped groove 43 is formed by etching the wafer 41 with a KOH solution through a window formed in a dielectric film 45 covering the surface of the silicon wafer 41. In this case, positions of the V-shaped groove 43 and the bonding pads 42 are set in advance so that the positions of the optical output portion of the optical semiconductor device and the V-shaped groove 43 coincide with each other. Then, the optical fiber 37 is placed in the V-shaped groove 43 and fixed there, once the silicon wafer 41 has been partitioned into chip-form sections and the p-side electrode 34 of the semiconductor laser 35 has been connected to the extending electrode 44 and bonded onto the chips, thus completing the coupling. Since the above-mentioned semiconductor laser 35 and optical beam diameter conversion waveguide 36 are isolated electrically from each other by means of the groove 32, a current for driving the semiconductor laser is not conducted into the optical beam diameter conversion waveguide 36, and moreover, the parasitic capacity of the semiconductor laser 35 is not increased. Besides, as shown in Fig. 9 the generated light can be absorbed if a small part 34A of the p-side electrode 34 proximate to the optical beam diameter conversion waveguide 36 is made separate from the remaining part of the electrode and a reverse bias is applied to the separate part 34A. Therefore, an electric field absorbing type optical modulator 40 may also be formed between the semiconductor laser 35 and the optical beam diameter conversion waveguide 36 by means of the separate p-side electrode 34A as shown in FIG. 9. Since the taper configuration in the mode converting region 17 of the MQW layer of the device is obtained by defining the shape of the mask which is used for growing a film, without changing the etching process, crystal defects are less likely to be produced in the part of MQW layer having the tapered form. As a result, less adverse effects on the characteristics arise from such crystal defects in this layer. Moreover, the taper configuration can be formed with small error and high yield, thus making it possible to obtain beam spots having a uniform configuration. (The second embodiment) In the first embodiment described above, a mask having a spatula-shaped window 18 such as shown in FIG. 3A is used in order to vary the film thickness of the semiconductor layers 19 to 23 forming the optical beam diameter conversion waveguide 36. Further, in Fig. 3A the waveguide forming region 17 of the window 18 has the above-mentioned trapezium shape which widens uniformly at a fixed angle, but alternatively, as shown in Fig. 14, a window 38 having a pattern that widens in stepwise manner may also be formed by forming portions 38A to 38D where the width is not changed and spreading portions 38E to 38H alternately. In this way, the film thickness of the semiconductor layers 19 to 23 of the window optical beam diameter conversion waveguide 36 becomes thinner stepwise toward the bottom end portion as shown in FIG. 15, and the characteristic thereof is almost the same as that of the first embodiment. Besides, since it is preferable to make the steps produced in the thickness direction of the semiconductor layers 19 to 23 as small as possible, it is desirable that the width of the window 18 of the mask is changed by as many steps as possible. In a region where it is desired to make the differences in the coefficient of optical absorption larger, the variation of the thickness of the semiconductor layers may be made larger by making the variation of the width larger. An example of a method of mass production of optical semiconductor devices in which a semiconductor laser and an optical beam diameter conversion waveguide are formed in a monolithic manner will be described next. For the purpose of mass production, a plurality of windows 38A are arranged at predetermined intervals in parallel with a dielectric film 16A in a configuration in which regions corresponding to the semiconductor beam forming regions 12 and optical beam diameter conversion waveguide regions 17 shown in FIG. 3A are connected in series repeatedly as shown in FIG. 16. Thereafter, a plurality of optical semiconductor devices each having a semiconductor laser and an optical beam diameter conversion waveguide are formed continuously on the substrate through a process such as that described hereinbefore with reference to the first embodiment. Thereupon, it is sufficient to cleave the substrate 11 along cut lines L 11 to L 14 and L 21 to L 26 so as to separate the individual optical semiconductor devices. (The third embodiment) The guide layers 19 and 21 are provided above and below the MQW layer 20 in the first embodiment described above, but a construction without these layers may also be adopted. The energy band thereof appears as shown in FIG. 17A and FIG. 17B. However, the well layer and the barrier layer become as thin as several nm near to the bottom end portion of the optical beam diameter conversion waveguide 36. Therefore, when the guide layers 19 and 21 are not provided, the difference of the refractive index in the width (lateral) direction may be too small and optical confinement becomes weaker. Therefore, a ridge construction in which an uppermost cladding layer 28 of the optical beam diameter conversion wavequide 36 is formed that in plan view has a configuration similar to that of the MQW layer 20, as shown in FIG. 18A and FIG. 18B is adopted. With this, both sides of the cladding layer 28 are exposed. Thus, a lateral mode is guided and optical confinement is improved. In order to form the cladding layer 28 in a ridge construction, as shown in FIG. 18A, a groove 32 for separating the optical semiconductor laser 35 from the optical beam diameter conversion waveguide 36 is filled with an insulating material 38 such as polyimide to be flattened, and then patterning is applied to the cladding layer 28 by a photolithography method with a dielectric film such as SiO 2 or SiN as a mask, thereby to form the ridge construction. An n-side electrode 33 and a p-side electrode 34 are formed thereafter as shown in FIG. 18B. Next, the coupling loss when an optical fiber 37, having a core diameter of 10 µm, and the bottom end of the above-mentioned optical beam diameter conversion waveguide 26 are placed directly opposite each other was computed, with the result shown in FIG. 19 which was better than the result obtained in the first embodiment shown in FIG. 11. (The fourth embodiment) In the optical beam diameter conversion waveguide 36 of the first embodiment, the cladding layers 15, 22 and 28 above and below the MQW layer 20 which becomes the core layer are made of the same material as that of the semiconductor laser 35, but cladding layers 15A and 22A may be formed of a material such as InGaAsP having a refractive index smaller than that of the cladding layer of the semiconductor laser 35 as shown in FIG. 20. According to the above, optical confinement in the optical beam diameter conversion waveguide 36 is reduced thus making it possible to make the optical beam diameter in the film thickness direction larger. Furthermore, when the cladding layers 15A and 22A are non-doped, light absorption by free carriers is reduced. Besides, an InGaAsP etching stop layer 23A is provided between the guide layer 21 and the cladding layer 22, thus adjusting the etching depth when the cladding layers 22 and 28 located in the optical beam diameter conversion waveguide 36 are removed, and an upper non-doped InGaAs cladding layer 22A is formed after the etching. (The fifth embodiment) A semiconductor laser is used as an active element connected to the optical beam diameter conversion waveguide in the first embodiment described above, but a photodiode may also be adopted. When a reverse bias is applied to the semiconductor laser 35, the element thereof functions as an end face wave conductive type photodiode of a pin junction construction. Further, a photodiode of pn junction and an end face light emitting type light emitting diode of pn junction or pin junction may also be used as an optical active element. In this case, althouqh it is not illustrated specifically, the construction of the core layer of the optical beam diameter conversion waveguide connected to the photodiode or the light emitting diode is the same as that of the optical absorption layer or the active layer of the optical active element. Further, when the photodiode is formed in one body with the optical beam diameter conversion waveguide, it may also be arranged to form a recessed portion in a photodiode forming region and to form a photodiode therein as shown in FIG. 21 for instance. The process thereof will be described briefly. First, after forming an n-InP buffer layer 52 and an InGaAsP etching stop layer 53 successively on an n-InP substrate 51, a dielectric mask having an almost spatula-shaped window similar to those shown in FIG. 3A and FIG. 15 for instance is used, and an n-InP cladding layer 54, an InGaAs core layer 55 and a non-doped InP cladding layer 56 are formed on the etching stop layer 53 through that window. At this time, the core layer 55 and the upper and lower cladding layers 54 and 56 are thick in the region of a photodiode 57 and get thinner in the direction toward the bottom end portion of an optical beam diameter conversion waveguide 58. This is due to the same principle as that of the first embodiment. Thereafter, those semiconductors located in the forming region of the photodiode 57 among the semiconductors on the etching stop layer 53 are removed so as to form a recessed portion 59, and an n-InP layer 60, an i-InGaAs layer 61 and a p-InP layer 62 are laminated one after another therein thereby to form the pin junction portion of the photodiode 57. Then, finally, a p-side electrode 63 is formed on the p-InP layer 62, and an n-side electrode 64 is formed under the n-InP substrate 51. When light which has propagated through the optical beam diameter conversion waveguide 58 is incident on the i-InGaAs layer 61 of the photodiode 57 in a state in which reverse bias is applied to the photodiode 57 having such a construction, a current is produced between the electrodes 63 and 64. At this time, when the energy band gap of the core layers 55 located before and after the photodiode 57 in the direction of light propagation is set to a wavelength shorter than 1.3 µm at the optical wavelength conversion, the i-InGaAs layer 61 is thicker than the core layer 55 and the energy band gap becomes smaller, thus making it possible to use light having a wavelength band longer than 1.3 µm, and the coefficient of optical absorption and the optical confinement efficiency are improved, thus making it possible to obtain a high quantum efficiency with a short absorption layer. Besides, the photodiode 57 buried in the recessed portion 59 may be of an ordinary pn junction type by impurity diffusion. Now, as shown in FIG. 22, it is also possible to use an arrangement in which optical beam diameter conversion waveguides 58A and 58B are formed before and after the recessed portion 59 in the direction of light propagation, respectively. Optical signals λ 1 +λ 2 +λ 3 having a plurality of wavelengths are inputted to the waveguide 58A on the input side, and only the signal λ 1 of a specific wavelength is removed by the photodiode 57 formed in the recessed portion 59, the remaining signals λ 2 and λ 3 being outputted from the waveguide 58B on the output side. In this case, it is required to select as the material of the core layer of the photodiode 57 a semiconductor having an energy band gap suitable for removing only the light having a specific wavelength. When the light λ 2 , λ 3 is passed through the photodiode 57, the light spreads in the width direction through the core layer 55 of the optical beam diameter conversion waveguide 58B, and optical confinement gets weaker in the thickness direction of the core layer with greater proximity to the end portion. Thus, the optical beam diameter gets large. Further, when the light λ 1 , λ 2 , λ 3 enters the core layer 55 of the optical beam diameter conversion waveguide 58A, the optical confinement becomes more intense with greater proximity to the photodiode 57, and the optical beam diameter get smaller. (The sixth embodiment) FIG. 23 shows an optical semiconductor device in which a travelling wave type laser amplifier and an optical beam diameter conversion waveguide are formed in a monolithic manner. This device is obtained by forming an n-InP buffer layer 62 and an n-InP cladding layer 63 on the n-InP substrate 61, forming an active layer (core layer) 64 structured of MQW composed of i-InGaAs and i-InGaAsP thereon, and forming thereafter a p-InP cladding layer 65 and a p-InP contact layer 66 at the center thereof. Further, an n-side electrode 67 is formed under the n-InP substrate 61, and a p-side electrode 68 is connected to the contact layer 66. In this optical semiconductor device, respective semiconductor layers above the n-InP cladding layer 63 are thickest at the central region thereof, and get thinner towards the end portions in respective regions before and after the central region. Further, the two regions in which semiconductor layer film thicknesses vary become optical beam diameter conversion waveguides 69A and 69B, and a semiconductor laser amplifier 70 is formed by a semiconductor layer in the central region thereof. The variation of the thickness of the semiconductor film is achieved by epitaxial growth using another mask having a window showing a narrow width at the center having a shape looking like two masks such as shown in FIG. 3A and FIG. 14 put together. In this case, since not only is the coupling good between the semiconductor laser amplifier 70 and the optical beam diameter conversion waveguides 69A and 69B but also the beam diameters at the exposed end, portions of the optical beam diameter conversion waveguides 69A and 69B become larger. Hence, the coupling efficiency with optical fibers 71 and 72 is high. With this, the total optical amplification factor becomes larger. Besides, a p-side electrode 68 may also be formed on the optical beam diameter conversion waveguides 69A and 69B. Now, a pin junction is formed between the semiconductor laser amplifier 70 and the active layer (core layer) 64 of the optical beam diameter conversion waveguides 69A and 69B in the optical semiconductor device shown in FIG. 23, but, in an alternative embodiment in which a pn junction of the active layer 64 of the laser amplifier 70 is formed, the pn junction is also formed for the active layer 64 of the optical beam diameter conversion waveguides 69A and 69B. Besides, this semiconductor laser amplifier 70 is driven by applying a positive bias to the electrodes 67 and 68. Since light is absorbed when a reverse bias is applied, however, it is also possible to use the optical semiconductor device shown in FIG. 23 as an electric field absorbing type optical modulator. When the optical semiconductor device such as shown in FIG. 23 is mass-produced, it is sufficient to form a dielectric film 16A having windows 38A as shown in FIG. 16, to form semiconductors selectively on the windows 38A, and to cleave the semiconductors along the lines L 11 and L 13, and L 21 , L 22 , L 23 , L 24 , L 25 and L 26 thereafter. Besides, in the first to the sixth embodiments described above, the light propagation loss of the semiconductor is reduced by either forming no electrode on the top side of the optical beam diameter conversion waveguide or non-doping a semiconductor forming a cladding layer. When impurities are doped in the cladding layer, however, if the thickness of doping is restricted as small as possible, absorption by free carriers is reduced and the light propagation loss is reduced. Further, when the above-mentioned optical semiconductor device is designed in an optimal manner, a maximum coupling efficiency of 90% or higher is obtainable, and in addition, large values of 5 µm in a vertical (longitudinal) direction and 6 µm in a horizontal (lateral) direction are obtainable as a coupling tolerance against lowerina the coupling efficiency by 3dB. (The seventh embodiment) FIG. 24 is a perspective sectional view showing a Fabry-Pérot semiconductor laser according to an embodiment of the present invention. This semiconductor laser has a construction such that a gain peak appears in a 1.3 µm band, and the construction will be described along the manufacturing process. First, as shown in FIG. 25A, a first mask 84 having an opening over at least a gain region 79 and a mode conversion regic 80 is formed on an n-type (n - ) InP substrate 81. The mask 84 is obtained by patterning an insulating film such as a silicon nitride film by photolithography, and the configuration of the opening 85 of the mask 84 has a rectangular portion 85a having a length of 400 µm measured in a light travelling direction in thegain region 79, a trapezium portion 85b having a length of 350 µm and a width Which increases uniformly at a constant angle with greater distance from the base of the rectangular portion 85a, and an extended portion 85cextending further from the base of the trapezium portion 85b. Next, as shown in FIG. 25B (and also FIG. 26A which shows a sectional view taken along a line III-III shown in Fig. 25B), an n-InP cladding layer 82, a first SCH layer 83 composed of In x Ga 1-x As y P 1-y (0<x<1, 0<y<1), an MQW layer 86, a second SCH layer 87 composed of In x Ga 1- - x As y P 1-y , and a p-InP cladding layer 88 are grown, one after another, epitaxially on an n-type InP substrate 81 exposed from the first mask 84. As to the film thicknesses of the different layers, the n-InP cladding layer 82 is grown to a thickness of 100 nm, the first and the second SCH layers 83 and 87 are grown to a thickness of 100 nm, and the p-InP cladding layer 88 is grown to a thickness of 200 nm. Crystals of these layers are not grown on the mask 84 which has no growth nucleus, but are grown selectively on the n-type InP substrate 81 located in the opening portion 85 of the mask 84. Moreover, in these layers, the film thickness is thickest in the stripe-shaped gain region 79 of the opening portion 85 of the mask 84, and the film thickness gets smaller with increasing distance from the gain region 79 in the mode converting region 80 extending therefrom. Such a method of growing a film is called a selective area growth method. The thickness of well layers 86a of the MQW layer 86 at the bottom end portion of the mode converting region 80 is 1/3 of the film thickness in the gain region 79. This MQW layer 86 functions as an active layer in the gain region 79, and serves as a waveguide in the mode converting region 80. Since the MQW layer 86 is formed in a tapered shape in the mode converting region 80, that region may also be referred to as a tapered region hereinafter. The MQW layer 86 is made up of three well layers 86a composed of In x Ga 1-x , As y , P 1-y , (0<x'<1, 0<y'<1) and barrier layers 86b composed of In x ,, Ga 1- - x ,,As y ,, P 1-y ,, (0<x"<1, 0<y"<1) sandwiched therebetween, and the thickness of the well layer 86a is 7 nm and the thickness of the barrier layer 86b is 15 nm in the gain region 79, respectively. After the above-mentioned first mask 84 is removed, a second mask 89 is formed on the p-InP cladding layer 88 and extends in the light travelling direction as shown in FIG. 27A which is a sectional view taken along a line IV-IV in FIG. 25B. The mask 89 is obtained by applying patterning to a silicon nitride film thereby to form it in a rectangular stripe shape 1.5 µm in width. At this time, the beam spot is formed in a circular or nearly circular shape with the ratio of the minimum values of the stripe widths in the gain region 79 and the mode converting region 80 set to 0.8 to 1.2. Thereafter, the p-InP cladding layer 88 to the upper part of the n-InP cladding layer 82 are shaped by etching in an almost perpendicular direction using the second mask 89 as shown in FIG. 27B. Then, a first p-InP buried layer 90 and an n-InP buried layer 91 are formed one after another on the n-InP cladding layer 82 located on both sides of the second mask 89 as shown in FIG. 27C, without removing the second mask 89. Next, after the second mask 89 is removed, p-InP is laminated on the p-InP cladding layer 88 and the n-InP buried layer 91, thereby to increase the thickness of the second cladding layer 88 and also to form a second p-InP buried layer 92 on the n-InP buried layer 91 so as to form a current blocking layer 93 (see Figure 24) having a thyristor construction on both sides of the MQW layer 86. Furthermore, p+ type InGaAsP is laminated on p-InP as shown in FIG. 26B, which is used as a contact layer 94. This contact layer 94 is made not to cover the tapered region 80 either by using a mask for preventing crystal growth or by applying patterning to the semiconductor layer. Thereafter, an insulating film 95 is formed on the contact layer 94, the buried layer 92 and the second cladding layer 88, and an opening portion 96 is formed only above the MQW layer 86 of the gain region 79 by applying patterning to the insulating film 95 by photolithography. After completing lamination of compound semiconductor and the insulating film in this manner, a p-side electrode 97 is formed on the contact layer 94 and an n-side electrode 98 is formed on the under surface of the n-InP substrate 81 as shown in FIG. 26C. Furthermore, both side faces of the InP substrate 81 and the semiconductor layers formed on the substrate 81 are cleaved. Besides, a reflecting film may also be formed on the cleaved surface of the gain region 79. FIG. 24 shows a partly-cut-away perspective view of a Fabry-Perot semiconductor laser formed by the process as described above. Since the taper configuration in the mode converting region 80 of the MQW layer of this semiconductor laser is obtained by defining the shape of the mask, which is used for growing a film, without changing the etching process, crystal defects are less likely to be produced in the tapered part of the MQW layer. As a result, adverse effects on the characteristics due to crystal defects are not produced in this layer. Moreover, the taper configuration is formed with small error and high yield, thus making it possible to obtain beam spots having a uniform configuration. In the MQW layer 86 of this semiconductor laser, the light which has oscillated in the gain region 79 resonates with the reflection from the end face of the gain region 79 and the end face of the mode converting region 80 and is outputted from the end face on the side of the mode converting region 80. In this case, the potential at the ground level of the quantum well becomes larger as a well layer 86a gets thinner owing to a quantum size effect, and the band gap between the conductive band and the valence band of the well is also expanded. Since a band gap Eg 11 in the gain region 79 shown in FIG. 28A is smaller than a band gap Eg 22 in the mode converting region 80 shown in FIG. 28B, the MQW layer 86 in the mode converting region 80 transmits the light oscillated in the gain Since the variation of the film thickness of the MQW layer 86 in the tapered region 80 is gentle, however, a part of the tapered MQW layer 86 that is near the gain region 79 may become a saturable absorption layer. When a saturable absorption layer is in existence, the light is absorbed until the layer is saturated by the carriers. Therefore, the threshold gets higher and a kink is produced in the optical output vs. current characteristic (I-L characteristic) curve, thus making sufficient light intensity no longer obtainable. Thus, as shown in FIG. 29, it is preferable to modify the structure such that the p-side electrode 97 and the contact layer 94 in the gain region 79 of the semiconductor laser extended to a saturable absorption region 78 in the vicinity of the tapered region 80, and a current is also injected into the MQW layer 86 of the saturable absorption region 78. According to such a structure, a current is also injected to the MQW layer 86 of the saturable absorption region 78 through the electrode 97, and light absorption in that region of the MQW layer 86 is countered by injection of carriers, so that the layer 86 does not become a saturable absorption layer, but becomes transparent and transmits the light. With this, a kink is no longer produced in the I-L characteristic curve, thus obtaining a characteristic such as shown in FIG. 30. According to the I-L characteristic curve shown in FIG. 30, a threshold current value of 19mA, which is lower than that in the constructions shown in citations [3] and [4], has been obtained with an element having double cleavage, and the differential quantum efficiency was as high as 0.25 mW/mA. Further, since an edge of the optical absorption vs. wavelength characteristic is shifted toward the short side of the oscillation wavelength (the side having lower wavelengths than the oscillation wavelength) and almost no saturable absorption region is generated in the mode converting region 80, neither hysteresis nor kinks are generated along the L-I curve, and it has been confirmed that reflection or radiation of light at the junction portion of the active region 79 and the tapered region 80 in the MQW layer 86 is controlled to be sufficiently low. Further, as the result of observation of a far field pattern, it has been found that the full width at half maximum (FWHM) of an output beam is 11.8° in the film thickness (vertical) direction and 8.0° in the lateral (horizontal) direction of the film as shown in FIGs. 31A and 31B, thus obtaining a beam output angle narrower than before. Further, when the relationship between the beam spot diameters in a vertical direction and a horizontal direction and the length (taper length) of the tapered region 80 in a near field pattern was investigated, a relationship such as shown in FIG. 32 was obtained. In this case, the thickness of the well layer 86a of the MQW layer 86 in the tapered region 80 was set to 1/5 of the thickness of the well layer 86a in the gain region 79. In FIG. 32, the mode of the light cannot follow the change of the film thickness and adequate functioning as a mode converter is not obtainable when the length is such that the film thickness changes suddenly, i.e., the taper length is 50 µm or less. On the other hand, changes in film thickness which are suitable for proper functioning as a mode converter can be provided when the taper length is 50 µm or more even if the change of the spot diameter is gentle. Further, although it is not illustrated specifically, no change is noticed in the spot diameter when the taper length reaches 500 µm or more, and lengths longer than 500 µm therefore are unnecessary. Accordingly, it may safely be said that the optimum taper length is 50 µm or more and preferably 50 to 500 µm. When the ratio of the film thickness at the start to the end of the tapered region (bottom end) of the MQW layer 86 is two times or more, a function as a mode converter is obtainable. Next, an optimum magnitude of the change of the MQW layer 86 in the mode converting region 80 will be described. FIG. 33 shows a result of calculating how the beam output angle from the output end is changed when the film thickness ratio between the input end and the output end (input end film thickness/output end film thickness) of the mode converting region 80 of the MQW layer 86 that becomes the tapered waveguide is changed from 1 to 5. A normal three-dimensional beam propagation method is used for computation, which has been made on an MQW waveguide of a 1.3 µm band. As shown in FIG. 34, the film thickness of the MQW layer 86 at the input end is set to 0.1 µm, the thickness of the SCH layers 83 and 87 is set to 0.1 µm, and the length ℓ of the waveguide is set to 200 µm, and a loss-less waveguide in which change of the film thickness is uniform has been assumed. The lateral width of the tapered waveguide was set constant at 1 µm over the whole length of the waveguide. In FIG. 33, a square dot shows an emission angle of a beam in a horizontal direction with respect to the substrate surface, and a rhombic dot shows an emission angle of a beam in a vertical direction with respect to the substrate surface. According to FIG. 33, only an emission angle close to 30 degree is obtainable similarly to a general semiconductor laser when the film thickness ratio is set to 1. The emission angle is narrowed as the film thickness ratio becomes larger, and the angle reaches 15 degrees and 12 degrees when the film thickness ratio is 3 and 4, respectively. Further, the fact that the values of emission angles in the horizontal direction and the vertical direction for the same ratio become closer to one another as the ratio increases shows that the beam configuration becomes nearly circular, and a desirable result for the coupling with an optical fiber is obtained as the beam configuration gets nearer to a circle. When coupling with an optical fiber is considered, it is when the beam output angles both in the horizontal and vertical directions become half of or less than heretofore was the case, i.e. 15 degrees or less, that a large improvement in the coupling efficiency is made. The foregoing corresponds to the case in which the film thickness ratio is 3 or higher. Accordingly, when a mode converter (a spot size converter) is used, it is apparent from FIG. 33 that the film thickness ratio of the MQW layer 86 is preferably 3 or higher. However, even when the emission angle is 20 degrees, i.e., the film thickness ratio is 2, an improvement can be expected in the coupling efficiency owing to the fact that the beam gets nearer to a circle than a conventional laser and the mode configuration of the laser becomes close to the mode configuration of the optical fiber. Now, an optical modulator integrated light source in which the film thickness of the MQW layer is changed has been set forth in Japanese Patent Provisional Publication No. 1-319986 and Japanese Patent Provisional Publication No. 3-284891. In the former publication, it is proposed to change the film thickness of the MQW layer at the boundary between the gain region and the modulation region by changing the width of a mesa-stripe shape of the substrate. Further, in the latter publication, it is proposed to change the film thickness of the MQW layer at the boundary between the gain region and the modulation region by changing the stripe width of the mask. In these publications, the film thickness in the modulation region of the MQW layer is formed constant, and neither any description or suggestion is given with respect to the formation of the MQW layer into a tapered shape. In a light source with which an optical modulator is integrated, the MQW layer of the optical modulation region may be made of a composition having a shorter wavelength as compared with the composition of the MQW layer of the gain region. For example, when the wavelength of the composition of the gain region is 1.3 µm, the composition formed in the optical modulation region may be of wavelength 1.20 µm to 1.22 µm. Besides, when the composition of the gain region has the wavelength 1.5 µm, the composition in the optical modulation region may be of wavelength approximately 1.40 µm to 1.45 µm. When the above figure is converted into the film thickness ratio of the MQW layer, the film thickness ratio falls within the range of 1.5 to 2. When the composition is formed to have a film thickness ratio larger than the above, sufficient light absorption becomes unachievable unless the voltage for optical modulation is increased since the composition wavelength in the optical modulation region is too high. As a result, the working voltage of elements has to be raised. Practically, the film thickness of the MQW layer in the gain region of the integrated light source and optical modulator described in those patent provisional publications is up to two times the film thickness of the MQW layer of the optical modulation region. Accordingly, in a light source with which an optical modulator is integrated, it is desirable that the change of the film thickness of the MQW layer is smaller, and it thus differs from an effective change quantity of the film thickness of a semiconductor laser having an integrated mode converter embodying the present invention. As described above, it has been found by the present inventor et al. that the effective ranges of the change of the film thickness are different as between an optical modulator integrated light source and an optical mode converter integrated light source, and, in a light source having an integrated mode converter, it is required to make the film thickness of the waveguide layer of the mode converter two times or more at the input end against the output end, and a circular beam is obtainable by making the film thickness ratio five times. As described above, proper functioning as a mode converter is obtainable by making the film thickness ratio between the start and the end of the tapered region (bottom end) of the MQW layer 86 larger than two. Further, the change of the film thickness does not need to be uniform, and the film thickness may change suddenly in the vicinity of the gain region 79 and change gently with greater distance therefrom, or conversely may change gently in the vicinity of the gain region 79, or the rate of change of the film thickness may be changed gently a plurality of times. Further, the film thickness thereof may also be changed gently stepwise. Now, if it is desired to inject into the saturable absorption region 78 a current smaller than that injected into the gain region 79, two p-side electrodes 97 and 99 may be formed separately in the gain region 79 and the saturable absorption region 78 respectively as shown in FIG. 35. With this, a current injection quantity in the saturable absorption region 78 becomes less and power consumption is reduced. Furthermore, since the amount of light absorption in the saturable absorption region 78 of the tapered MQW layer 86 gets smaller gradually with increasing distance from the gain region 79, a smaller current value for eliminating optical absorption will suffice with increasing distance from the gain region 79. Accordingly, it is sufficient, instead of extending the p-side electrode 97, to extend just the contact layer 94 from the gain region 79 to the saturable region 78 as shown in FIG. 36. According to the foregoing, the contact layer 94 composed of an impurity-containing semiconductor has a higher resistance than a p-side electrode 97 made of metal, and a current injected into the MQW layer 86 through the contact layer 94 gets smaller with increasing distance from the gain region 79. Hence, it is possible to control supply of an excessive current. A Fabry-Perot construction has been adopted in the present embodiment, but a DBR laser construction may also be adopted by providing a diffraction grating above or below the MQW layer 86 of the mode converting region 80. Further, a diffraction grating may be provided along the MQW layer of the gain region so as to obtain a DFB laser construction. Further, the MQW layer is structured with InGaAsP/InP or InGaAs/InP group materials in the above mentioned embodiment, but an MQW formed of an GaAs/AℓGaAs or AℓGaAs/AℓGaAs group material, an GaInP/AℓGaInP or AℓGaInP/AℓGaInP group material or other compound semiconductor materials may also be used. Besides, the growth of the compound semiconductor layer may be performed by MOCVD, MBE or other epitaxial growth methods.

Claims: An optical semiconductor device in which an active optical element (35; 57; 70) and a waveguide (36; 58; 69A; 69B) are formed integrally, the device including: a support structure (11, 13-15; 81-83) having a semiconductor substrate (11; 81); and a quantum well structure (20; 86) formed on the said support structure and having a stack of layers including at least one well layer (20a; 86a) and two barrier layers (20b; 86b) sandwiching the well layer (20a; 86a), the said quantum well structure being formed in an active optical region (12; 79) of the said active optical element (35; 57; 70), and also being formed in a waveguide region (17; 80) that has a first end adjacent to the said active optical region (12; 79) and that has a second end adjacent to an interface portion of the device at which an output optical beam is emitted from the device, or an input optical beam is received by the device, when the device is in use, the quantum well structure (20; 86) in the said waveguide region (17; 80) forming at least part of a core layer (20; 86) of the said waveguide (36; 58; 69A, 69B) and serving to couple the active optical element (35; 57; 70) optically to the said interface portion when the device is in use, there being upper and lower cladding layers (15, 22/28; 82, 88) above and below the core layer (20; 86) respectively; characterised in that the thickness of the said well layer (20a; 86a) decreases gradually over the length of the said waveguide region (17; 80) from the said first end thereof to the said second end thereof. A device according to claim 1, wherein the said quantum well structure (20; 86) includes a plurality of such well layers (20a; 86a) and a plurality of such barrier layers (20b; 86b), the well layers (20a; 86b) and barrier layers (20b; 86b) being stacked alternately one upon another. A device according to claim 1 or 2, wherein the barrier layers (20b; 86b) decrease in thickness gradually from the first end to the second end of the waveguide region (17; 80). A device as claimed in any preceding claim, wherein in the waveguide region (17; 80) the quantum well structure (20; 86) increases in width gradually from the first end to the second end. A device according to any one of claims 1 to 3, wherein the quantum well structure (86) has substantially a stripe shape over the active optical region (79) and over the waveguide region (80), and the ratio of the minimum width value of the quantum well structure over the active optical region (79) and the minimum width value of the quantum well structure over the waveguide region (80) is in the range from 0.8 to 1.2. A device as claimed in any preceding claim, wherein the overall thickness of the quantum well structure (20; 86) at the first end is at least twice that at the second end. A device as claimed in any preceding claim, further comprising: a first guide layer (19; 83) formed between the quantum well structure (20; 86) and the said lower cladding layer (15; 82); and a second guide layer (21; 87) formed between the quantum well structure (20; 86) and the said upper cladding layer (22; 88); said first and second guide layers (19, 21; 83, 87) having an energy bandgap which is larger than that of the quantum well structure (20; 86) and lower than the respective bandgaps of the upper and lower cladding layers (15, 22; 82, 88). A device as claimed in claim 7, wherein the first and the second guide layers (19, 21) reduce in thickness gradually, in the portions thereof that extend above or below the said waveguide region (17) of the quantum well structure, in the direction from the first end to the second end. A device according to any preceding claim, wherein a diffraction grating (13) for forming a distributed feedback mirror is further formed under the active optical region (12; 79) of the quantum well structure (20; 86). A device according to any preceding claim, wherein the said upper cladding layer (22, 28) extends over the optical active region (12) as well as over the waveguide region (17; 80) of the quantum well structure and has a groove (32) to separate that layer into two parts, one of which parts extends over the optical active region (20a) and the other of which part extends over the waveguide region (20b). A device according to claim 10, wherein said groove (32) in the said upper cladding layer (22, 28) is filled with an insulating material (38). A device as claimed in any preceding claim, having a first electrode (33; 98) formed on a lower main face of the said substrate (11; 81) and also having a second electrode (34; 97), formed over the said upper cladding layer (22, 28; 88), and extending over at least the said active optical region (12; 79) of the quantum well structure. A device according to claim 12, further comprising a contact layer (94) formed between the said second electrode (97) and the said upper cladding layer (88). A device according to claim 13, wherein the contact layer (94) extends beyond the active optical region (79) so as to be over a part (78) of the waveguide region (80) as well. A device according to claim 14, wherein the second electrode (97) also extends beyond the active optical region (79) so as to be over the said part (78) of the waveguide region (80). A device according to claim 13, wherein the second electrode (97) and the contact layer (94) are both separated, in register with one another, into two parts where they extend over a boundary region between the active optical region (79) and the waveguide region (80). A device according to claim 7, wherein the or each well layer (20a; 86a) is made of InGaAs or InGaAsP, the barrier layers (20b; 86b) are made of InGaAsP, the first and the second guide layers (19, 21; 83, 87) are made of InGaAsP, and the semiconductor substrate (11; 81) and the upper and lower cladding layers (15, 22/28; 82, 88) are made of InP. A device according to any one of claims 1 to 17, wherein the or each well layer (86a) is made of InGaAsP of a first composition ratio, and the barrier layers (86b) are made of InGaAsP of a second composition ratio which is different from the first composition ratio. A device according to any one of claims 1 to 16, wherein the or each well layer (86a) is made of GaAs, and the barrier layers (86b) are made of AlGaAs. A device as claimed in any one of claims 1 to 16, wherein the or each well layer (86a) is made of GaInP and the barrier layers (86b) are made of AlGaInP. A device according to any preceding claim, further comprising buried layers (25; 93) formed on both sides of the quantum well layer (20; 86). A device according to any preceding claim, wherein a lower main face of the quantum well structure (20; 86) extends substantially parallel to an upper main face of the semiconductor substrate (11; 81). A device as claimed in any preceding claim, wherein said active optical element is a semiconductor laser (35), a semiconductor laser amplifier (70), an end-face light emitting type light emitting diode, a waveguide type photodiode (57) or a semiconductor optical modulator. A device as claimed in any preceding claim, wherein said upper and lower cladding layers (15, 22) are formed of a semiconductor material that is not doped with impurities. A method of manufacturing an optical semiconductor device in which an active optical element (35; 57; 70) and a waveguide (36; 58; 69A; 69B) are formed integrally, including: forming a masking film (16; 84) of dielectric material on an upper main face of a support structure (11, 13-15; 81) including a semiconductor substrate (11; 81); patterning the masking film (16; 84) so as to expose respective first (12; 85a) and second (17; 85b) regions of semiconductor material (15; 81) of the said support structure, the said first region (12; 85a) corresponding to an active optical region (12; 79) of the said active optical element (36; 58; 69A; 69B) and being elongate, and the said second region (17; 85b) corresponding to a waveguide region (17; 80) and increasing gradually in width from a first end thereof adjoining one end of the said first region (12; 85a) to a second end thereof adjacent to an intended interface portion of the finished device at which, in use of the said finished device, an output optical beam is to be emitted from the device or an input optical beam is to be received by the device; and forming layers (19 to 23; 86 to 88) successively over the first and second regions (12, 17; 85a, 85b) so as to provide over both regions a quantum well structure (20; 86) having a stack of the layers including at least one well layer (20a; 86a) and two barrier layers (20b; 86b) sandwiching the well layer (20a; 86a), the quantum well structure (20; 86) over the said second region forming at least part of a core layer of the said waveguide (36; 58; 69A; 69B), and so as to provide over at least the said second region (17; 85b) upper and lower cladding layers (15, 22/28; 82, 88) above and below the said quantum well structure (20; 86) respectively, at least the said well layer (20a; 86a) being formed by a selective area growth method so that its thickness decreases gradually over the length of the said second region from the said first end thereof to the said second end thereof. A method as claimed in claim 25, wherein the or each well layer (20a; 86a) and each barrier layer (20b; 86b) is formed by the said selective area growth method so that the thickness of both the well layers and barrier layers decreases gradually over the length of the said second region. A method according to claim 25 or 26, wherein the second region (17; 85b) is shaped substantially like a trapezium the two parallel sides of which are at the said first and second ends respectively of the second region (17; 85b). A method according to claim 27, wherein the width of the trapezium, as measured between the two non-parallel sides thereof, changes in stepwise manner from the said first end to the said second end. A method according to any one of claims 25 to 28, wherein the said semiconductor material (15) of the said support structure (11, 13-15; 81-83) serves in the finished device as the said lower cladding layer. A method according to any one of claims 25 to 29, wherein the said semiconductor material of the support structure is provided by a semiconductor layer (15) formed above the said semiconductor substrate (11), and the said first and second regions of semiconductor material exposed in the step of patterning the masking film form part of that semiconductor layer (15). A method according to any one of claims 25 to 29, wherein the first and second regions of semiconductor material exposed in the step of patterning the masking film form part of the said semiconductor substrate (81). A method according to any one of claims 25 to 31, wherein the layer forming step includes the steps of: forming a first such barrier layer (20b; 86b) above the first and second regions of the said semiconductor layer (15; 83); forming such a well layer (20a; 86a) on the first barrier layer (20b; 86b); and forming a second such barrier layer (20b; 86b) on the well layer (20a; 86a). A method as claimed in any one of claims 25 to 32, further comprising the steps of: removing the masking film (16; 84) from the said support structure; forming a first electrode (33; 98) under the support structure (11, 13-15; 81-83); and forming a second electrode (34; 97) above the upper cladding layer (28; 88) so that the electrode extends over the first region (12; 85a). A method according to claim 33, further comprising a step of forming a contact layer (94) between the upper cladding layer (88) and the second electrode (97). A method according to any one of claims 25 to 34, further comprising a step, after the layer forming step, of patterning the formed layers (19-23) into a stripe shape where they extend over the said first region (12), before completing the upper cladding layer (28). A method according to any one of claims 25 to 34, further comprising a step, after the layer forming step, of patterning the formed layers (86-88) into a stripe shape where they extend over both the first and second regions (85a, 85b).

Inventors: EKAWA MITSURU, FUJII TAKUYA, KOBAYASHI HIROHIKO, OGITA SHOUICHI, OKAZAKI NIROU, SODA HARUHISA, TABUCHI HARUHIKO, EKAWA, MITSURU, FUJII, TAKUYA, KOBAYASHI, HIROHIKO, OGITA, SHOUICHI, OKAZAKI, NIROU, SODA, HARUHISA, TABUCHI, HARUHIKO, EKAWA, MITSURU, C/O FUJITSU LTD., FUJII, TAKUYA, C/O FUJITSU LTD., KOBAYASHI, HIROHIKO, C/O FUJITSU LTD., OGITA, SHOUICHI, C/O FUJITSU LTD., OKAZAKI, NIROU, C/O FUJITSU LTD., SODA, HARUHISA, C/O FUJITSU LTD., TABUCHI, HARUHIKO, C/O FUJITSU LTD.
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1994
Main classification: H01S3
IPC codes: G02B6, H01S5